Electrostatic discharge (ESD) is a continuing problem in the design, manufacture, and utilization of integrated circuits (ICs). A major source of ESD exposure to ICs is from the human body (described by the “Human Body Model”, HBM). In this situation, a packaged IC acquires a charge when it is touched by a human who is electrostatically charged (e.g. from walking across a carpet). A charge of about 0.4 uC may be induced on a body capacitance of 100 pF, for example, leading to an electrostatic potential of 4 kV or more and discharge peak currents of several amperes to the IC for longer than 100 ns. A second source of ESD exposure is from charged metallic objects (described by the “Machine Model”, MM), which is characterized by a greater capacitance, lower internal resistance and transients that have significantly higher peak current levels than a HBM ESD source. A third source of ESD exposure is due to the discharge of stored charge on the integrated circuit itself (described by the “Charged Device Model”, CDM), to ground with rise times of less than 500 ps. The current flow during CDM is in the opposite direction of the HBM and MM ESD sources. For all three sources of ESD exposure, both positive and negative polarity discharges may occur.
A typical shallow trench isolation (STI) ESD protection diode which shorts the I/O pin to ground during an ESD event is shown in FIG. 1. The anode 114 which may be connected to Vss or ground is connected by P+ contact diffusion 104 to a pwell 102. The cathode 112 which is typically connected to an input/output (I/O) pin is connected to an N+ diffusion 106. The N+ diffusion 106 forms the STI ESD protection diode to the pwell 102. The pwell is typically formed on a more lightly doped p-type substrate 100.
During an ESD strike, the ESD diode may become forward biased shorting the input pin to Vss and shunting the ESD current to ground thus protecting the integrated circuit from damage. The time it takes for the ESD diode to turn on is proportional to the length of the current path 116. Because of the length of the current path 16 under the STI 124 during a fast rise time ESD event, a significant voltage overshoot 150 may occur as shown in FIG. 5.
To prevent ESD voltage overshoot a gate spaced ESD diode (GS ESD diode) such as is shown in FIG. 2 may be used. Because the current path 118 in the GS ESD P/N diode is shorter than the current path 116 of the STI ESD P/N diode, the turn on is significantly faster. The faster turn on time of the GS ESD diode avoids voltage overshoot as shown by the graph 152 in FIG. 5.
A typical shallow trench isolation (STI) ESD N/P protection diode which shorts the I/O pin to Vdd is shown in FIG. 3. The anode 122 which typically is connected to an I/O pin is connected to a diode formed between P+ diffusion 104 and nwell 130. The cathode 120 which typically is connected to Vdd is connected to the N+ diffusion 106 nwell 130 contact. The nwell 130 is typically formed in the p-type substrate 100.
A gate spaced (GS) ESD protection N/P diode which shorts the I/O pin to Vdd is shown in FIG. 4. In the GS ESD N/P diode a gate 110 and gate dielectric 108 electrically isolated the anode 122 from the cathode 132. The anode 122 is connected to P+ diffusion in the nwell. The cathode 120 is shorted to the gate 110 and is connected to the N+ 106 nwell 130 contact. The ESD protection diode is formed between the P+ diffusion 104 and the nwell 130.
A problem with the STI ESD P/N diode that is even more pronounced with the GS ESD P/N diode is capacitance. As shown in Table 1 the capacitance of a gate spaced ESD P/N diode (bottom wall capacitance 118, plus sidewall capacitance 120 plus gate overlap capacitance 116) may be more than twice the capacitance of a STI spaced ESD diode (bottom wall capacitance 126 in FIG. 1). The additional capacitance that the STI spaced or gate spaced ESD protection diode adds to the I/O pin may reduce the performance of the IC and may limit the maximum frequency at which the IC may be operated.
TABLE 1MeasuredVoltageCapacitanceovershootTYPE(fF)(V)STI Diode859Gate spaced diode1910